Purified perikarya from large ventral spinal neurons can be isolated in bulk fractions, using an adaptation of a method originally devised for neurons from brain. The isolation procedure, presently under development in our laboratory, includes homogenization of collagenase-treated ventral grey matter from bovine spinal cord, isopycnic centrifugation to obtain a crude neuronal fraction, and selective removal of contaminants and small neuronal perikarya by sieving with nylon bolting cloth of appropriate mesh size. Forty to seventy-five thousand neuronal cell bodies which are greater than 50 micron in their smallest diameter can be prepared in one day from 30 to 40 g wet weight of trimmed ventral enlargements. We propose here to complete the development of the fractionation technique, extending it to the isolation of large neurons from cat spinal cord, and then to investigate the chemical composition and ultrastructure of neurons obtained from eachtype of animal. This isolation procedure should significantly augment existing methods for studying the biochemistry of mammalian spinal neurons.